There is a large volume of literature related to ultrasonic flow meters. U.S. Pat. Nos. 2,874,568 and 2,923,155 describe ultrasonic flow meters in which the probes do not pierce the conduit wall, but rather are mounted external to the wall. With these arrangements the angle of refraction of the longitudinal wave at the interface between the conduit and fluid is relatively small and hence the ultrasonic waves have only a very minor velocity component in a direction parallel to the axis of fluid flow. With such systems, sensitivity, particularly to low flow rates, is a significantly limiting factor. In U.S. Pat. No. 3,575,050, the flow meter employs ultrasonic waves in the longitudinal mode transmitted diagonally across the conduit such that there is a significant component of the velocity in a direction parallel to the axis of flow of the fluid. This is achieved with transducers mounted external to the conduit and hence there is no perturbation of the flow of the fluid stream. The transducers include a generator/receiver of shear mode ultrasonic waves which is coupled to the conduit such that the shear waves are obliquely incident on the interface between the conduit and the fluid. These shear waves are mode converted by refraction at the interface into longitudinal waves, which are now transmitted at a much higher angle, typically twice that achievable with an incident longitudinal wave, through the fluid material. For measurement of the flow rate, the difference in transit time between a longitudinal wave travelling downstream and one travelling upstream provides the basis for a determination of the average flow rate. This determination of transit time may be made, for example, by clocking the time difference between two pulses or alternatively by measuring the phase shift between emitted continuous waves or bursts of continuous wave energy. Since the ultrasonic waves being transmitted through the fluid have a larger component of velocity in a direction parallel to the axis of flow, the sensitivity of the measurement is increased.
In a commercially available ultrasonic flow meter (M-2000 Series, Malema Sensors, Boca Raton, Fla., USA), two ultrasonic transducers are mounted so that the ultrasonic waves travel in parallel with the axis of the flow cell channel. Accordingly, the ultrasonic waves travel in parallel with the flow direction, so that the sensitivity of the measurement is maximized. There are further advantages related to the use of such a relative positioning of the transducers of ultrasonic waves, and the fact that the ultrasonic waves travel in a direction in parallel with the axis of the flow cell channel. For a traditional “clamp on” ultrasonic flow meter, the angle of refraction of the longitudinal wave at the interface between the conduit and fluid is relatively small and hence the ultrasonic waves have only a very minor velocity component in a direction parallel to the axis of fluid flow. U.S. Pat. Nos. 2,874,568 and 2,923,155 describe ultrasonic flow meters in which the probes do not pierce the conduit wall, but rather are mounted external to the wall. With these arrangements, as the material of the conduit wall, or pipe wall, is different from the fluid flowing within the pipe, the different acoustic properties of the two materials will cause an angular deflection of the beam of ultrasonic radiation according to Snell's law. Accordingly, this deflection will vary according to which fluid is flowing within the pipe. This may not be a problem for flow measurement in systems where the fluid composition remains essentially the same over time, for example in oil supply pipes, or for measurement of drinking water supply. In such systems, the deflection angle according to Snell's law will remain relatively constant over time. However, in systems for analysis, purification or preparation of bio molecules like proteins and nucleic acids, such as by the use of chromatography or filtration (for example by the use of membranes), the fluid composition may change significantly even within the time of a single separation experiment. In chromatography, solvent and salt gradients are used on purpose to speed up separations and to obtain improved resolution of the separation. Typical examples of solvent gradients include raising the concentration of an organic solvent (such as methanol or acetonitrile) from 0% at the start of the gradient to 80% at the end of the gradient. Typical examples of salt gradients include raising the concentration of a salt (such as NaCl) from 0 M at the start of the gradient to 2 M at the end of the gradient. The gradient time can be the full run time of the experiment, or just a part of the run time of the experiment. Due to the geometric arrangement of traditional “clamp on” ultrasonic flow meters, the apparent measured flow rate will then differ as the composition of the fluid is changed. However, this effect will be minimized with a flow meter arrangement wherein the ultrasonic transducers are axially aligned, positioned opposite each other and in axial alignment with the flow cell channel, because the angle of refraction of the longitudinal wave at the interface between the conduit (or pipe) and fluid is 90 degrees and, additionally, the ultrasonic waves velocity component is mainly in a direction parallel to the axis of fluid flow.
However, the ultrasonic flow meters described in the literature (e.g., U.S. Pat. Nos. 2,874,568; 2,923,155 and 3,575,050, and the published specifications for ultrasonic flow meters from Malema Sensors) are not suitable for systems used for analysis, purification or preparation of bio molecules like proteins and nucleic acids, for example by the use of chromatography or membrane filtration. Thus, despite the frequent use of flow meters in separation systems, such as chromatography systems, there are no publications recommending the use of ultrasonic flow meters in such systems. These systems require better than 5% accuracy at all flow rates within the range 25 ml/min up to 9 L/min. Furthermore, the backpressure from the flow cell should be less than 0.4 Bar for flow rates ranging from 25 ml/min up to 9 L/min, and the bandbroadening effect of the flow meter must be less than 10% for chromatographic separation, operating at flow rates within the range 25 ml/min to 9 L/min. Preferably the material of the flow cell should be characterized by low damping of ultrasonic waves, in order to increase the sensitivity. More preferably, the material should also be suitable for disposable or single-use systems, for example, it should be relatively inexpensive, sterilisable and ideally combustible.
U.S. Pat. No. 5,463,906, for example, discloses a disposable flow cell assembly which serves as an acoustic chamber for use with an ultrasonic flowmeter for the measurement of blood flow. Acoustic-coupling bodies having an acoustic impedance similar to blood, which is different to the acoustic impedance of the plastic from which the bulk of the flow body is normally constructed, are used to avoid substantial refraction or reflection regardless that the angle between the acoustic vector and the wall(s) of the flow chamber is not a right angle. The presence and design of these acoustic coupling bodies also prevents the production of eddies or whirlpools which might otherwise damage the blood cells or induce clots. While the flow cell assembly described in U.S. Pat. No. 5,463,906 is designed to measure the flow rate of blood it would not be suitable for use in measuring the flow rate where fluid compositions may change, such as in chromatography and filtration applications.
There is a significant interest in disposable systems, in particular for use in strictly regulated processes, for example separation or purification of chemicals, bio molecules or other components for use in pharmaceutical applications. Materials used in such systems preferably fulfil the requirements of United States Pharmacopeia (USP) class VI to guarantee that they do not release harmful substances during use. Such systems should also provide a sterilised environment in order to meet the strict requirements for such applications. Accordingly, it should be possible to sterilise the system, for example by the use of gamma irradiation. Sterilisation is here construed to mean a reduction in microbial population or load. However, most users do not have the possibility to perform gamma irradiation sterilisation in their laboratories. Further, most users do not want to perform gamma irradiation sterilisation in their laboratories, due to the potential health hazards associated with such processes. Therefore, they will prefer to buy validated sterilised disposable equipment from commercial suppliers. While it is possible to sterilise equipment by the use of antimicrobial agents in solution, this requires the use of a multi step cleaning procedure and this is a disadvantage.
Another disadvantage associated with prior art ultrasonic flow meters is that on exchange or replacement of flow meters, recalibration is required because the internal diameter of the flow cell channel varies considerably from flow cell to flow cell. As will be understood by the skilled person, this is particularly a problem for “clamp-on” ultrasonic flow meters which are attached to existing pipes (e.g. oil supply pipes) or conduits. This is a time consuming exercise which may be acceptable for flow cells which are to be used for long periods of time but is not acceptable for disposable and/or sterilised flow cells which may only be used for a single run or a short period of time.